Preparation Of Nanocellulose-based Aerogels And Its Application Research In Oil-water Separation

Chemical oil leakage and organic waste liquid discharge bring pressure to the ecological environment and threaten the survival of human beings.The use of lightweight and highly porous aerogels as adsorption materials to solve the problem of marine oil pollution has become a research hotspot.Nanocellulose aerogel is a new green renewable three-dimensional porous material,which has the characteristics of biodegradability,large specific surface area,rich in hydroxyl groups,and strong reactivity,and is an ideal oil-absorbing material,but because of its hydrophilicity and poor mechanical stability,it is not conducive to oil-water separation.In this paper,three different methods were used to prepare nanocellulose,and nanocellulose aerogels were prepared by freeze drying.Then,hydrophobic nanocellulose aerogels were prepared by hydrophobic modification to achieve oil-water separation.Finally,polyvinyl alcohol（PVA）was crosslinked with nanocellulose to prepare composite nanocellulose aerogels and hydrophobic modification to improve the mechanical strength and service life of nanocellulose aerogels.The main research contents are as follows:Firstly,nanocellulose was prepared using different methods.Nanocellulose（expressed as CNF,ECNF and TCNF）was prepared by high-pressure homogenization,enzyme pretreatment-high-pressure homogenization,and TEMPO oxidation pretreatment-high-pressure homogenization using bleached softwood pulp as raw materials,and the effects of different preparation methods on the yield,structure and properties of nanocellulose were studied.The results showed that the yield of nanocellulose preparedby TEMPO oxidation pretreatment-high-pressure homogenization method was 92.4%,which was higher than that of the other two preparation methods,and the size distribution of TCNF was more uniform,with no obvious flocculation and agglomeration phenomenon occurred in the filaments,and the dispersion in aqueous solution was more stable.There was no obvious difference in the chemical structure of CNF,ECNF and TCNF,and the crystal forms were all celluloseΙ,and the degree of crystallinity were 82.0%,78.8%and 77.3%,respectively.TCNF was poorer in low-temperature thermal stability,and degradation occurring earliest at210°C,while CNF has the smallest residue of 27%at 500°C.Secondly,hydrophobic nanocellulose aerogels were prepared by hydrophobic modification.Hydrophobic nanocellulose aerogels（expressed by S-CNFA,S-ECNFA,S-TCNFA）were prepared by freeze-drying and hydrophobic modification with stearyl chloride,using the prepared CNF,ECNF and TCNF as raw materials,and the structure and properties of nanocellulose aerogels before and after modification were analyzed.The results showed that the prepared nanocellulose aerogels are stacked from cellulose sheets,has a spatial structure with high porosity and low density,and the porosity can reach 99.5%.Among the three nanocellulose aerogels,the structure of TCNFA is more complete and the pore distribution is more uniform.Compared with the unmodified nanocellulose aerogel,the density of the modified nanocellulose aerogel increased,the porosity decreased,and the surface water contact angle was more than 130°,which showed good hydrophobicity.Among them,S-TCNFA had the best adsorption for n-hexane,vacuum pump oil,edible oil and silicone oil,and the adsorption capacity reached 26.9 g·g^-1,45.4 g·g^-1,48.9 g·g^-1and 52.6 g·g^-1,respectively.After multiple cycles oil absorption test for S-TCNFA,the adsorption capacity of the fifth cycle was60.8%of the first cycle.Finally,in order to improve the mechanical strength of nanocellulose aerogels,TCNF/PVA composite aerogels were prepared by in-situ composite using crosslinking agent PVA and TCNF,and hydrophobic TCNF/PVA composite aerogels were prepared by freeze-drying and hydrophobic modification with stearoyl chloride.The results showed that the density of the modified composite aerogel is 15.2 mg/cm³and the porosity is 99.0%when the TCNF:PVA is 1:0.5,which can meet the requirements of high porosity and low density of aerogel,and the surface water contact angle of the modified composite aerogel is 150.4°,showing the superhydrophobic state.With the increase of PVA,the internal structure of aerogel becomes tighter and the porosity decreases.FTIR analysis showed that the-OH content in the composite aerogel increased with the increase of PVA,and the C=O content in the modified composite aerogel increased.The-OH in the PVA molecule forms a hydrogen bond with the cellulose surface-OH,which increases the bonding between the molecules,increases the mechanical strength of the TCNF/PVA composite aerogel,and increases the recyclability of the aerogel.At the same time,the-OH in PVA molecules also provides more esterification grafting points for SAC,which can introduce more SCA and increase the hydrophobicity of aerogels.After three cycles of extrusion,the elastic recovery rate of aerogel can still reach 64.5%,and after five cycles of oil absorption,the adsorption capacity of the fifth time can reach 31.56 g·g^-1,which is 69.7%of the first adsorption capacity,showing good recyclability performance.