Synthesis Of Composites Based On Alkali/urine Solution System And Their CO₂ Capture

Secondary fibers reused for papermaking cause fiber hornidicaiton due to repeated wetting and drying,thereby the swelling capacity and plasticity of fibers are lower.In theory,a hornified fiber can be recycled 6–7 times,while in reality,a hornified fiber is recycled only 2.4 times on average in global,which means secondary fibers will still be discharged into the environment as municipal solid waste.Therefore,it is imperative to investigate how to repair a hornified fiber for the purpose of improving the swelling ability of the fiber and to develop its high-value utilization for changing the fate of eventually becoming municipal solid waste,which is a challenge for science and technology researchers working in waste paper recycling.Irreversible hydrogen bonding is one of the most widely recognized mechanisms for inducing fiber hornification,and development of easy-gained,low-cost and high-capacity CO₂ capture materials is urgent issue for improvement of CO₂ capture and storage technology.Herein,we propose to treat hornification fibers with Na OH/urea aqueous solution that can disassemble inter-and intramolecular hydrogen bonds,in order to improve the swelling ability of the fibers by splitting the irreversible hydrogen bonds between the fiber microfibers,meanwhile use this solution as a solvent for dissoving cellulose to prepare waste paper base cellulose solid adsorbent for CO₂ capture,providing a new way for the high-value utilization of waste pape.The main research results were as follows:1.Hornified fiber treated with a Na OH/urea aqueous solutionAfter the hornified fibers were treated with the Na OH/urea aqueous solution,the lignin content decreased,the fine fiber content increased,and the filling of the cellulose film between the interfiber network provided an increased number of hydrogen-bonding binding sites;in addition,the ordered crystal structure of cellulose was deteriorated;all of these resulted in the easier swelling of the fiber and subsequent improvement in the water retention value(WRV);the cellulose film filled in the fiber network and formed a structure that was similar to a reinforced concrete structure,thereby increasing the paper strength.Under the optimal condition of a pulp(dry)concentration of 10%,sodium hydroxide concentration of 1%,urea concentration of 8%,solution pre-cooling temperature of 0°C,soaking time of 12 h,a post-treatment medium of 7%(NH₄)₂SO₄,a stirring speed of 1500 rpm,a stirring time of 30 min,the WRV of the obtained pulp fiber and the tear,burst,and tensile index of the resulting paper sheets increased by 54%,277%,394%and 98%,respectively.2.Synthesis of SiO₂/cellulose composite porous material and its characterization of CO₂ adsorptionA fly-ash-based SiO2 wet gel and waste-paper-based cellulose were used as raw materials for the in situ synthesis of a series of silica/cellulose composite porous materials using the Na OH/urea solution by a“one step”method.The structures of the composite material were characterized by FTIR spectroscopy,FESEM,XRD,TG/DSC,and N₂ adsorption/desorption isotherms.The results showed that this method can successfully synthesize SiO₂/cellulose composites,in which SiO₂ and cellulose complexes bound by physical interactions.The silicon-to-cellulose ratio of2.5:1 was the critical concentration for the dominant skeleton structure of the composite porous material.Below this concentration,the skeleton structure of composite materials comprised cellulose filaments with spherical nanosilica particles decorated on their surface,and their physical properties(such as crystal structure,thermal properties,microstructure and pore size,etc.)were determined by the cellulose skeleton structure;above this concentration,the dominant skeleton structure of composite materials comprised spherical nano-SiO₂ particles with cellulose filaments embedded in the skeleton structure,and the properties of the material(such as crystal structure,thermal characteristics,micro-morphology,and pore size,etc.)were determined by the SiO₂ skeleton structure.A series of SiO₂/cellulose porous materials was investigated for CO₂ adsorption under ambient conditions.The CO₂capture capacity decreased in the following order:CA-Si-0>CA-Si-0.5>CA-Si-1>CA-Si-1.5>CA-Si-2>CA-Si-2.5>CA-Si-3.The full-cellulose porous material CA-Si-0,i.e.,without Si addition,exhibited an excellent CO₂ capture capacity of 3.68mmol/g.3.Synthesis and characterization of holocellulose aerogels and its application for CO₂captureBy using the Na OH/urea solution as the solvent,a series of holocellulose aerogels CA-x was prepared by dissolving,regenerating,and freeze-drying of purified cellulose extracted from old corrugated containers.In addition,FTIR spectroscopy,XRD,FESEM,TG/DSC,N₂ adsorption/desorption isotherms,and others methods were performed to analyze their physical properies.The results showed that these hybrid processes do not change the chemical structure of the cellulose macromolecular chains,apart from the physical structure,such as the transition of cellulose type I to cellulose type II structure and deterioration of the thermal stability.Compared to purified cellulose,the as-synthesized CA-x materials comprised cellulose filaments with a diameter of less than 50 nm as an irregular honeycomb three-dimensional porous network structure,constituting mesoporous materials with type IV isotherms with H3 hysteresis loops.The pore size was about 5–20 nm,the specific surface area was 132.72–245.19 m²/g,and the pore volume was 0.73–1.53cm³/g.Under ambient temperature and pressure,the material exhibited superior CO₂adsorption capacities in the range of 1.96–11.78 mmol/g,and its adsorption capacity decreased in the order of CA-2>CA-3>CA-1>CA-4>CA-5>CA-6>CA-7.4.Synthesis and characterization of amine-impregnated composites and its application for CO₂ captureUsing waste-paper-based cellulose aerogels(CAs)as the carrier,monoethanolamine,diethylenetriamine,tetraethylenepentamine,polyethylenimine and polyethyenepolyamine were immobilized by physical impregnation.N₂adsorption/desorption isotherms,TG/DSC,SEM,and FTIR spectroscopy were employed to characterize its structure,and its CO₂ adsorption properties were investigated.The results showed that the mesopores of the CA carrier are filled with amines as non-porous materials due to the intervention of amines,and the thermal stability of the composite material deteriorates.From the perspectives of thermal stability and CO₂ adsorption capacity,polyethylenimine was the best choice for the physical immobilization of CAs for CO₂ capture.At an adsorption temperature of100°C,the PEI30@CAS sample,i.e.,with a polyethylenimine loading of 30%,can capture up to 3.11 mmol/g of CO₂.