Preparation And Characterization Of Lignin-Based Carbon Nanotubes

As a typical representative of one-dimensional fibrous carbon materials,carbon nanotubes have been widely used in nano-manufacturing technology,electronic materials and devices,biomedical materials,composite materials,energy storage materials and other fields due to their unique physical and chemical properties.Up to now,scientists have discovered many methods for synthesizing carbon nanotubes,including arc discharge method,chemical vapor deposition method,laser ablation method,electrolysis method,etc.Lignin is a renewable biomass resource with abundant reserves,which has the characteristics of non-polluting and wide sources.In this paper,enzymatically hydrolyzed lignin(EHL)was used as the carbon source,various lignin/catalyst precursors were designed,and lignin-based carbon nanotubes(L-CNTs)were prepared by solid-phase pyrolysis and chemical vapor deposition.EHL and the catalyst were made into dry-blend powder,film,electrospinning fibre,and nano-micelle of lignin and the catalyst,respectively,by means of the mechanical mixing method,solution casting method,micelle template method,and electrostatic spinning method,with EHL as the carbon source under the principle of catalytic chemistry.Then the products were pyrolyzed in the tube furnace with the solid-phase pyrolysis method to prepare L-CNTs and composite amorphous carbon.The final product was obtained through purification in the mixed acid(volume ratio:98%concentrated sulfuric acid:65 concentrated nitric acid=3:1).The carbon materials synthesized by the precursor mixture by the micelle template method showed ideal morphology and property,which is the key objective of the research.In the precursor mixture of the catalyst,agglomeration of catalyst would inactivate the catalyst.By measuring the hydrodynamic diameter and PDI of lignin/micelle catalyst,influence factors of the particle size and particle size distribution of the nano-micelle were studied,including solvent type,water droplet acceleration,initial concentration,and water content.The optimal processing to prepare precursor nano-micelle of lignin/micelle catalyst were:NMP as organic solvent,initial concentration of lignin of 5 mg/mL,water content of 80%,and water droplet acceleration of 120 mL/h.The structure and properties of the self-made carbon nanotubes were characterized by Raman spectroscopy and transmission electron microscopy,and the effects of catalyst type,catalyst content,and pyrolysis temperature on the synthesis of carbon nanotubes were investigated by comparing their morphology and quality,so as to select the optimal synthetic process for carbon nanotube and prepare precursor nano-micelle of lignin/micelle catalyst with the micelle template method.In the process,the mass fraction of Fe(NO3)3 was 6%and was given solid-phase pyrolysis at 800℃ to obtain carbon nanotube/composite amorphous carbon with good crystal.In this research EHL was used for the first time as carbon source to prepare L-CNTs with mobile chemical vapor deposition,and a preparation method different from the traditional hard template cytalyst method was proposed.The iron salt solution was first added to the lignin solution in the ratio of 4:1 to self-assemble into spherical lignin micellar nanospheres,and the lignin micellar nanospheres were dispersed on the silicon wafer by impregnation,roasted in air to remove the organic shell on the surface of the iron salt and reduced to monolithic iron nanoparticles by H2,and then EHL was pyrolyzed in an inert gas atmosphere to synthesize carbon nanotubes on the surface of the iron-based catalyst using the resulting small molecule gas.The micellar template method has effectively solved the aggregation problems of the catalyst during preparation,and the generated catalyst particles had even size and good dispersion,and suitable to be the precursor to synthesize L-CNTs.The effects of growth temperature,flow rate of carrier gas,catalyst type,and catalyst concentration on the morphology and structure of L-CNTs were also investigated,and L-CNTs with diameters between 10 nm～80 nm,lengths between 1μm～4 μm and rich functional groups were obtained.Optimal preparation processing of L-CNTs was selected to prepare the precursor of catalyst based on the micellar template method and impregnation method.EHL was pyrolyzed at 400℃,then made into L-CNTs based on the mobile chemical vapor deposition method at 900℃ and flow rate of carrier gas of 200sccm.On this basis,pyrolysis analysis of EHL was combined to describe the growth process of L-CNTs.This research provides a novel preparation method for catalyst and a synthetic method for L-CNTs,and offers a new reference to the high valued application of industrial lignin.