Preparation Of Cotton-based Activated Carbon Fibers And Research On Characteristics And Performance

Activated carbon fiber(ACF)is an activated carbon converted from textile materials.ACF is equally a product of the fusion of textile manufacturing and polymer thermochemical process technologies.Thanks to ACF’s unique pore structure and fiber morphology,it has excellent adsorption,electrical,catalytic,thermal and so on.Moreover,because of the textile state,it is more advantageous in terms of loading,storage,and transportation than powder and granular activated carbon.Currently,the largest category of commercially available ACF products is viscose-based ACF.As environmental protection requirements become more stringent,the low-cost advantages of traditional viscose fibers continue being weakened.At the same time,the accumulation of waste cotton textiles also means that huge low-cost cellulose resources are yet to be developed.Therefore,the cost barrier of using cotton fiber as a raw material for ACF production may be broken.However,at present,there are limited studies on Cotton-based ACF,which leads to a state of under-preparation in both basic research and industrial practice.There are many problems in the correlation between the preparation process and structural features and the impact on performance that need to be recognized and solved.In view of this,the preparation and influencing factors,the new preparation process,the relationship between the preparation conditions and the structural characteristics of the product,the comparison between the Cotton-based ACF and the viscose-based ACF,and the application properties of the Cotton-based ACF are of great significance.In this paper,the influence of the preparation process on the physical properties of Cotton-based ACF is taken as the starting point.Uniform design method and the single factor experiment method were used to investigate the effects of heating temperature,phosphoric acid concentration and carbonization time on the structure and yield of Cotton-based ACF.The influence of various factors on the Cotton-based ACF pore structure is heating temperature>phosphoric acid concentration>carbonization time,while the yield of the product is mainly affected by the heating temperature.The mechanism of action of phosphoric acid in Cotton-based ACF preparation was clarified by optical microscopy image analysis,thermogravimetric analysis and STA-GC/MS simultaneous thermal analysis.Phosphoric acid plays a multiple role in the preparation of Cotton-based ACF,mainly promoting hydrolysis,catalytic dehydration,cross-linking,promoting aromatization and pore formation.Established preparation conditions are:heating temperature 600℃,phosphoric acid concentration 50 wt.%,carbonization time 2 h.In this paper,microwave technology was used to explore the new preparation process of Cotton-based ACF,which confirmed the feasibility of preparing ACF under microwave.The carbonization and activation of cotton under microwave using SiC are in line with the strategy of converting microwave energy by high-efficiency microwave absorbing materials.The effects of five factors including carbonization temperature,flame retardant concentration,activation temperature,CO2 flow rate and activation time on pore structure and yield were investigated by orthogonal experiment.The possible process principle is that the SiC container converts the microwave energy into heat energy and transfers it to the cotton to complete the pyrolysis in the carbonization process.In addition to the thermal energy transferred by SiC conversion during activation,the carbon fiber exposed to microwave irradiation can also be heated by microwaves,which itself becomes a microwave absorber to provide the thermal energy of the Boudouard reaction.The experimental conditions are as follows:carbonization time 550℃,(NH4)2HPO4 concentration 8wt.%,activation temperature 600℃,CO2 flow rate 25mL/min,activation time 30min.Investigations into the chemical structural characteristics of Cotton-based ACF include elemental composition,surface functional groups,and atomic structures.The main technologies used are X-ray energy spectroscopy(EDS),X-ray photoelectron spectroscopy(XPS),Fourier transform infrared spectroscopy(FT-IR),transmission electron microscopy(TEM),X-ray diffraction(XRD)and Raman spectroscopy(Raman)et al.The most important elemental composition of Cotton-based ACF is C,and a small amount of O and possibly residual P,and is basically consistent with the viscose-based ACF.Both FT-IR and XPS analysis indicated the presence of oxygen-containing groups in addition to the C=C bond on the Cotton-based ACF surface.TEM analysis showed that the microstructure of the Cotton-based ACF was disordered in the main body of the lamellar structure,while the partial regions were at an ordered state.The the interplanar spacing of(002)and(100)of the Cotton-based ACF were calculated to be 3.8 A and 2.1 A,while and grain sizes to be 9 A and 12.5 A,respectively.There are more obvious D and G peaks in the Raman spectrum of Cotton-based ACF.The fitting of the Raman spectrum is suitable for the Lorentz equation.During the preparation process,an increase in temperature will cause a disordered enhancement of the Cotton-based ACF crystallite structure.The study also found that the XRD data of Cotton-based ACF has a certain correlation with Raman spectroscopy.Physical structure characteristics of Cotton-based ACF are mainly studied from physical morphology and pore structure.Scanning electron microscopy(SEM)was used to collect the Cotton-based ACF fabric structure,single fiber transverse and longitudinal topography images,and compared with a viscose-based ACF.Cotton-based ACF has a textile-like structure,and the transverse morphology of a single carbon fiber exhibits a waist-shaped and a meso-cavity structure,while the longitudinal morphology is twisted.The above-mentioned topographical features of Cotton-based ACF are comparable to those of cotton fiber,which is completely different from the groove morphology of the viscose-based ACF.For the study of the Cotton-based ACF pore structure,the N2 and CO2 adsorption isotherms and the hysteresis loop scan are mainly performed by the physical adsorber.From the measured isotherm data,the pore structure information including specific surface area,pore volume,specific pore surface area,micropore volume,and pore size distribution was obtained by analytical calculation of different models.The influence of the selection point method and the model on the calculation results in the analysis process is discussed.The Cotton-based ACF adsorption isotherm measured under the condition of N2@77K belongs to the I(b)type adsorption isotherm.It can be seen that the adsorption branch and the desorption branch separate to form a hysteresis loop phenomenon.The calculated BET specific surface area obtained by the optimized selection method is better than the fixed selection method,and it is necessary to examine the R2 of the linear fitting and ensure that the C>0 is a key indicator to promise the accurate result.It was noted that the Cotton-based ACF is suitable for the pore size distribution based on the nitrogen adsorption isotherm using the QSDFT model.Through the high-resolution pore size distribution map,it can be determined that the Cotton-based ACF has a rich pore structure,mainly micropores and ultramicropores,and has a small amount of mesopores,and the pore shape may be mainly a slit shape.The single H4 type hysteresis loop also shows that the Cotton-based ACF pore structure is mainly a narrow fracture hole,and there are structural features of the local area network communication and the channel blockage.The application and evaluation of the adsorption performance of Cotton-based ACF were carried out by dynamic benzene adsorption,water vapor adsorption and dynamic adsorption of dye KN-R in water.Adsorption behavior of different adsorbates on Cotton-based ACF in gas phase or liquid phase was studied,and the adsorption properties of Cotton-based ACF for several typical adsorbates were evaluated.Moreover,the relationship between adsorption performance and the structure of Cotton-based ACF pores and the possible adsorption mechanism was discussed.Experiments show that Cotton-based ACF has good dynamic benzene adsorption performance,and the dynamic benzene adsorption value is 828mg/g(293K,p/p0=0.175).There is a strong correlation between micropore volume and dynamic benzene adsorption value.The Cotton-based ACF has a strong adsorption capacity for water vapor,and equilibrium adsorption capacity is 693 cm3/g(273 K).The fractal dimension D analyzed by the FHH model is 2.75,which proves that the surface of the Cotton-based ACF is rough.Static water adsorption can reflect the BET specific surface area characteristics of ACF to some extent.The adsorption amount of Cotton-based ACF to KN-R is 28.4 mg/g.The adsorption isotherm of the Cotton-based ACF for the dye KN-R is consistent with the Langmuir model.The adsorption kinetics were better fitted by the Largergren pseudo-first order model.The lower initial dye concentration can reach the adsorption equilibrium faster,indicating that the Cotton-based ACF has an advantage in the low concentration of adsorbate in the liquid phase.