The Studies On The Molecular And Pore Structural Transitions And Novel Functional Applications Of Rayon Based-ACF

Rayon-based activated carbon fiber（RACF） has excellent adsorption and can be used widely in chemical industry,personal protection,pharmacy and hygiene,ect., and has shown the vast potential in the new fields of environmental protection and clean energy resources,too.But,the applications are hindered by its disadvantages such as complicated reactions in processing,low strength and yield of resultant, mono-disperse micro-pore,high electric consumption and cost,and the lack of practically technical development.In order to overcome these disadvantages,this paper has investigated systematically the catalysts,the mechanism of chemical and physical structural transition of rayon macromolecule in fabrication,pore structure control,low-cost processing technique,the adsorption and photo-catalytic degradation to organic composite pollutants,and high performance electrode,etc.With the help of differential thermogravimetric analysis（DTG）,5 catalysts, ammonium phosphate（AP）,dibasic ammonium phosphate（DAP）,ammonium dibasic hydrogen phosphate（ADP）,phosphoric acid（PA） and ammonium sulfate（AS）,have been used to treat rayon fibers and make ACFs in parallel to compare their reactive ability with pure rayon as a control.At the same time,reactive mechanisms have also been guessed.The catalysts can reduce the bond energy selectively,and then lows reactive temperature,dehydrates rayon molecule first and so increases char yield. DAP has the highest catalytic activity among the catalysts basing on the yields of carbonization and activation,as well as specific surface area of ACF.During the low temperature treatment,the air or oxygen is favorable to the dehydration and pyrolysis of rayon,and so enhances the yield when DAP is used.But there are no notable differences on catalyses of other 4 catalysts in or not in air.The relation between the shape character of the DTG peaks of rayon and pyrolytic speed as well as char yields has been built first in this paper,and thus a method to choosing an effective catalyst of rayon-base ACF has been founded.The main pyrolytic peaks on the DTG curves can be characterized by the ratio(H/W_1/2) of the heights（H） to the half-high widths(W_1/2) of the peaks.The little the value of the ratio,indicating the more moderate the pyrolytic reaction is,the higher the char yield. On the contrary,a bigger ratio shows a lower yield certainly.There is no clear relation found between areas of peaks and yields.From the ratio a pyrolytic behavior and char yield can be deduced rapidly for a new catalyst.The chemical,crystalline,pore structures and their transitional mechanism of rayon catalyzed by DAP during the stepwise heat treatment were researched by Fourier transform infrared spectroscopy（FTIR）,wide angle X-ray diffraction （WAXD）,elemental analysis（EA）,scanning electric microscopy（SEM）,atomic force microscopy（AFM） and automatic nitrogen apparatus,etc.DAP can promote the structure transformations evidently,lower the starting dehydration temperature of rayon to 150℃.Group C=O is generated at 200℃while group C=C appears at 250℃. These temperatures are lower 50-100℃than the corresponding ones of pure rayon. 220-250℃is a key temperature range where the chain,ring and crystal structures of rayon macromolecules are damaged completely to lose the basic characteristics of cellulose by dehydration,companying by the drastically physical changes:weight-loss, discoloration and shrinkage,etc.At this time rayon is changed into amorphous thermo-cellulose.It has been confirmed again by FTIR that oxygen in air indeed accelerates the dehydration and carbonization of rayon,and lows the corresponding reaction temperature 30℃at least.DAP enters and breaks down the crystal region of rayon partly,the crystal structure of rayon,however,can restore when DAP’s crystal is cracked in the 150-175℃.DAP can not only behave like an acid catalyst,but also react with rayon.Differing from former view,DAP works as both a catalyst and a reactant.The bands of chemical groups decrease very fast and disappear during carbonization.The exception is that the band of bond C=C exists still and transfers from 1715 cm^-1 to 1584 cm^-1 resulting from aromatization.At this stage,the diameter of rayon fiber keeps unchangeable but its weight-loss increases steadily.DAP does not notably impact on the structures and carbonized rate while the temperature is over 450℃.Very tiny crystalline structures of turbostratic graphite begin to form near 800℃,leading to a much broad diffuse peak on WAXD.The increment of usage of DAP can get higher yields of oxidation,carbonization and resultant ACF.The activation at a lower temperature（750℃） decreases the contents of oxygen-containing groups,but increases them at a higher temperature（950℃）.The widths of micro-pores on ACF concentrate on 2nm.The roughness and burn-off of ACF quite depend on the activation temperature.The surface of ACF can keep even when activation carries out below 850℃,but become rugged when over 950℃.A higher activation temperature can enhance the specific surface area and pore volume,but not increase the ratio of m（?）so-pore effectively.ACF made from rayon-based carbon fiber carbonized at a high temperature 1500℃has close cobble-like or elongated spherical structures on its surface,and pores are formed among the spherical structures.The sizes of spherical structures and pores range from 10nm to 25nm observed by AFM.These images are much different from pitch- or phenol-based ACF.The ordinary micro-pore ACF is limited to use in dealing with low molecular weight organs such as acetone,toluene,etc.But in some cases,middle molecular weight substances（such as dyes and organic pesticides） and macromolecules（such as detergents, drugs polymers,aerosols,bacteria and viruses,etc.）,need elimination by adsorption.Their sizes can reach from several to hundreds of nanometers,micro-pore ACF is not suitable for them.The pores must be widened to the same order of magnitude in order for better adsorption.Occasionally meso-pore ACF was mentioned but not in detail,The fabrication techniques of meso-pore and macro-poreACF were pursued innovatively to enhance the adsorption to the bigger molecules in the new fields.TiO₂ powder,as an additive,was added in the rayon solution to produce meso-pore ACF with two-step routine as usual.The volume of meso-pores rather relies on the temperature and time of activation.A higher activation temperature and longer time can give higher meso-pore volume and specific surface area,and in turn a lower yield.The ratio of meso-pore volume is up to 50.6%and the peaks of pore distribution locate at 2nm and 4nm,when activation at 950℃for 60min. By secondary activation,the micro-pores of common ACF can be widened to macro-pores with the width of 20-150nm and the ratio of meso-pore volume as high as 91.5%after the impregnation with composite cobalt salts.The fiber with macro-pores,not reported before, still keeps the satisfactory handle,strength and feasibility.The deeper the activation extent is,the bigger the pore width and volume and inter-planar spacing do,and the smaller the crystalline size and crystallinity are.Methylene blue,a dye with a middle molecular weight（volume）,was adopted to test the adsorption of ACFs with micro-pore,meso-pore and macro-pore respectively.The bigger average pore width shows a faster rate and higher capacity of adsorption.Therefore,ACF with macro-pores is much favorable for the organic macro-molecular substances.The common two-step routine（TSR）（carbonization and activation） is both time and energy consuming,and higher cost,too.A novel technique—single-step routine（SSR）（no carbonization and inert gas N₂,only direct activation）,has been investigated to make low-cost rayon-based ACF.Comparing with the ACF from TSR,ACF from SSR has the similar strength,handle,surface chemical and pore structure,but somewhat different crystalline size.In order to evaluate the two routines,a new term,effective yield（EY） （=activation yield x specific surface area）,was put forward to balance the two contradict technical indexes in ACF process.SSR is superior to TSR in the removal of carbonization and inert gas N₂.It decreases electric consumption and investment of device,finally enhances processing efficiency and reduces the cost by 1/3.Moreover,SSR has a higher EY and more steady shrinkage for resultant ACF.Lastly,RACF has been researched for the applications in environmental purification and clean energy material.The samples of RACF in form of non-woven felt were put in a small air-tight glass container to adsorb the different gases of single volatile organic solvents and multiple pollutants of macro-molecular organs in a solution,or formaldehyde in an imitated indoor air cabinet（2.2m³） or a 20m² room in-situ（full of new furniture）,respectively.It is very fast for ACF to adsorb the vapors of non-polar organs with low molecular weight such as acetone,tetrachloromethane, ethanol and toluene.Generally only about 10min is spent to reach equilibrium with the amounts adsorbed from 400 up to 1500mg/g.Regenerations are rather easily by heat treatment in air,and secondary adsorption volume keeps more than 95%of the first time.But it is much slow to adsorb polar molecules like steam,needing 2hr to reach saturated volume of 400mg/g.Formaldehyde is an exception and adsorbed hardly by ACF in room temperature,which comes from its very low boiling point （-19.5℃）.The adsorption to formaldehyde would be increased greatly to 450mg/g by the surface modification of ACF through loading organic amines.5 times of adsorption of activated carbon,ACF also exhibits an outstanding adsorption to the complex oil from different organics with low-and macro- molecules.For the cycling salt bath of PAN fiber processing,containing a concentration of 1.3%oil,when the usage of ACF is only 0.94%of the bath,95%oil in the bath can be removed at room temperature after adsorption only one time.Higher temperature to 60-90℃will lead higher adsorption.The soaked ACF by oil can be regenerated by organic solvent such as ether,with a desorption of 79%.The regenerated ACF shows a relatively lower secondary adsorption to oil more or less.Dealing with dilute organic macromolecular pollutants is very challenging,there are no good solutions now because of high cost and hard regeneration of filters（such as filtering film）.They can be enriched on ACF and then broken down to no-harmful substances by photo-catalytic TiO₂ loaded on it.Tetra-n-butyl titanate was as a precursor to prepare porous nano-anatase on the surface of ACF,via sol-gel method and stepwise heat treatment under 550℃.When an ultraviolet lamp is used to supply energy to excite TiO₂,70～80%citric acid or poly（vinyl alcohol）（PVA） in aqueous solution is decomposed completely within 6hr,comparing to the tens of hrs while only TiO₂ powder is used.ACF loaded TiO₂ can be used repeatedly in the cycle of adsorption--photo regeneration--adsorption,with each cycle at a cost of 1-2%specific surface area.The results testify that the technique of ACF combining photo-catalysis is feasible and promising to resolve the problems of macromolecular pollutants with high boiling points,as well as ACF’s regeneration.ACF has shown a great potential as a novel high performance electrode in super capacitor.ACF electrode must be of both high strength and specific surface area （1200m²/g） with a ratio of meso-pores of 30%at least.This electrode has not been supplied domestically,and not developed,either.In order to meet these strict requirements for mechanical processing and good electrochemical properties,a high strong rayon-based ACF has been trial-produced in batch,and used to fabricate electric double layer capacitors with capacity as high as up to 1×10⁴F and 8×10⁴F successfully.This special ACF was activated at a higher temperature（900-950℃） for longer time（40-60min）.Its technical parameters have reached top level in China with a high specific surface area of 1400-2000m²/g,a strength of more than 100N/5cm and a meso-pore ratio of 32.68%.Under imitated operating mode,the capacitors have been detected systematically by Arbin battery tester.When the testing currents were up to 50A and 100A,they still behaved very well with a very high specific capacity of 130—150F/g,near Russian top level.So they can be applied in auxiliary electric storage system of sonar power and wind-power,etc.In this paper,many important aspects on the fabrication and functional applications of RACF have been comprehensively investigated,innovative results and certain progresses have been obtained including the method for the evaluation of the catalysts,the processing techniques of macro-pore and high-strong as well as low-cost ACFs,the photo-catalytic decomposition to macro-molecular organics,etc.These results will give great help to understand the thermal reactive mechanism of rayon, enhance the strength and decrease the cost of RACF,and enlarge the application in new fields.